This invention relates to a hydrophone for measuring acoustic ambient noise in the ocean at low frequencies and more particularly to an inexpensive pressure balanced, high receiving sensitivity hydrophone.
Piezoelectric materials can be formed in a variety of shapes and be polarized in a number of directions, but the most common type of assembly is a ceramic tube electroded on its inner and outer surfaces and polarized in the radial direction. A radially poled tube can be waterproofed and can be used directly as a hydrophone. Such a hydrophone has no depth limit but it sensitivity is very low because the acoustic pressure acts on all surfaces and any charges (voltages) which are generated are proportional only to the change in volume. A preferred method is to acoustically shield the interior of the ceramic tube so that the acoustic pressure acts on only one surface and generates larger voltages. The interior shielding of the ceramic tubes can be accomplished in several ways. As an example, the interior surface can be covered with a "pressure releasing material" such as air containing foams. Alternatively, rigid end caps can be applied to the ends of the tube trapping a volume of air in the interior of the tube and thus shielding the interior of the tube. The sensitivity of the hydrophone using such a shielded ceramic tube is much higher than that of a hydrophone using an unshielded tube of the same geometry. However, the hydrophone using a shielded ceramic tube has a limited depth capability because the pressure releasing material such as foam gets compressed and the ceramic tube fractures in the air-filled device. If the ceramic tube is capped with rigid end caps and is filled with a fluid, the rigid end caps provide the necessary shielding function and the hdyrophone has a sensitivity approaching (but not equalling) the sensitivity of a hydrophone using an air-filled ceramic tube. The sensitivity loss is dependent upon the compliance of the fluid used for filling the ceramic tube and the dimensions of the tube. In order that the hydrophone be pressure balanced, it is necessary that one end cap be vented to a reservoir of the fluid filling the ceramic tube by means of an orifice or a capillary.
Thus pressure balancing in hydrophones can be achieved in many ways. One such technique is to use oppposed pressure relief valves and another one is to use a capillary tube. The use of opposed pressure relief valves is very effective in pressure balancing and results in a hydrophone which has good response characteristics at low frequencies and high pressures. However, this approach limits the size of the hydrophone which can be built since the internal diameter of a sphere or a tube used must be large enough to accomodate the pressure reilef valves. The most commonly used approach to pressure balancing is to use a capillary tube which is placed in one end cap of a rigidly end cap tube or is used to pierce the wall of a sphere. One such design has been described in U.S. Pat. No. 3,781,781 by Iver D. Groves. At very low frequencies the capillary tube provides a low acoustic impedance which allows balancing of the hydrostatic pressure. At some higher frequency, the capillary presents a high acoustic impedance and the hydrophone receiving sensitivity approaches to that of hydrophone having a totally enclosed sphere or a ceramic tube with rigid end caps. Irrespective of the use of a capillary or pressure relief valve, the receiving sensitivity of an oil-filled hydrophone having end cap tube or end-closed sphere is less than that which can be achieved for a hydrophone having an air-filled tube. The degree of departure from the ideal air-filled case is dependent upon the characteristic of the filling oil.
Besides the pressure balancing of the hydrophone, a capillary tube also limits the utility of the hydrophone at low frequency because the capillary looks like a low acoustic impedance at some low frequency. The low frequency cut-off, defined as the frequency at which the sensitivity decreases by 3 db is given by: EQU f=a.sup.4 d c.sup.2 /16.times.LV
where x is the viscosity of the oil used; L is the effective capillary length; V is the internal volume of the oil; a is the internal diameter of the capillary; d is the density of the oil and c is the speed of sound in the oil.
From the above relationship, it should be possible, in principle, to place the cut-off frequency as low as one would desire simply by properly choosing dimensions but there are physical limitationson the size of capillary which can be fabricated and assembled particularly in the case where small size and low frequency sensitivity are required. As an exmale, a hydrophone constructed of two end-glued cylinders each with an internal diameter of 1.3 cms and wall thickness of 1.6 m.m and of a length 1.3 cms filled with castor oil and using capillary 6.3 m.m long with an nternal diameter of 0.127 m.m would have a cut-off frequency of 25 to 30 Hz. This problem is further complicated by the fact that transfer phase considerations dictate that the frequency cut-off should be placed at least a decade below the lowest frequency at which operation is desired. It is thus desirable to have a high receiving sensitivity acoustic hydrophone which has a low cut-off frequency and which is pressure balanced, i.e., which is essentially insensitive to high static pressure.